MMUSIC D. Wing
Internet-Draft Cisco Systems
Expires: April 7, 2005 October 7, 2004
Symmetric RTP and RTCP Considered Helpful
draft-wing-mmusic-symmetric-rtprtcp-01
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as
Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 7, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document defines symmetric RTP and symmetric RTCP and recommends
their use.
Requirements Language
The key words "MUST", "MUST NOT" "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
Wing Expires April 7, 2005 [Page 1]
Internet-Draft Symmetric RTP and RTCP October 2004
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Definitions . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1 Symmetric RTP . . . . . . . . . . . . . . . . . . . . . . 3
2.2 Symmetric RTCP . . . . . . . . . . . . . . . . . . . . . . 3
3. Recommended Usage . . . . . . . . . . . . . . . . . . . . . . 4
4. Security Considerations . . . . . . . . . . . . . . . . . . . 4
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 4
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6.1 Normative References . . . . . . . . . . . . . . . . . . . . 4
6.2 Informational References . . . . . . . . . . . . . . . . . . 4
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 5
Intellectual Property and Copyright Statements . . . . . . . . 6
Wing Expires April 7, 2005 [Page 2]
Internet-Draft Symmetric RTP and RTCP October 2004
1. Introduction
Because RTP and RTCP are not inheriently a bi-directional protocols,
the usefulness of symmetry has been generally ignored. Many
firewalls, NATs [6], and RTP implementations expect "Symmetric RTP",
and do not work in the presense of non-symmetric RTP. However, this
term has never been defined. This document defines Symmetric RTP and
Symmetric RTCP.
TCP [3], which is inheriently bidirectional, uses symmetric ports.
That is, when a TCP connection is established from host A and its
source TCP port "a" to a remote host, the remote host sends packets
back to host A's source TCP port "a".
UDP isn't inheriently bidirectional and UDP itself doesn't require
similar port symmetry. Rather, some UDP applications (DNS [11]) have
symmetry, some UDP applications (TFTP [12]) don't have symmetry, and
other UDP applications (RTP [5]) don't mention symmetry.
2. Definitions
2.1 Symmetric RTP
The UDP port number for RTP media stream is usually communicated
using SDP [7]. The SDP is usually carried by a signaling protocol
such as SIP [8], SAP [9], or MGCP [10].
A device supports Symmetric RTP if, when receiving a bi-directional
RTP media stream on UDP port A and IP address "a", it also transmits
RTP media for that stream from the same source UDP port A and IP
address "a".
A device which doesn't support Symmetric RTP would transmit RTP from
a different port, or from a different IP address, than the port and
IP address used to receive RTP.
2.2 Symmetric RTCP
The advertisement of the UDP port number for RTCP is usually
communicated using SDP, and the port number is either implicit (RTP
port + 1, as described in RFC3550 [5] section 11) or explicit (as
described in Alternative Network Address Types [4]). The SDP is
usually carried by a signaling protocol such as SIP, SAP, or MGCP.
A device supports Symmetric RCTP if, when receiving RTCP for a media
stream on port B and IP address "b", it also transmits its RTCP
messages for that stream from the same source UDP port B and IP
address "b".
Wing Expires April 7, 2005 [Page 3]
Internet-Draft Symmetric RTP and RTCP October 2004
A device which doesn't support Symmetric RTCP would transmit RTCP
from a different port, or from a different IP address, than the port
and IP address used to receive RTCP.
3. Recommended Usage
There are two specific instances where symmetric RTP and symmetric
RTCP are required.
The first instance is NATs that lack integrated Application Layer
Gateway (ALG) functionality. Such NATs require the RTP endpoint use
UDP port symmetry to establish bi-directional traffic. ALGs are
defined in section 4.4 of RFC3022 [6].
The second instance is Session Border Controllers (SBCs) and TURN
[13] servers, which relay RTP media and RTCP packets. Media relays
are useful in conjunction with symmetric NATs to allow bi-directional
UDP traffic across such NATs. However, if the RTP endpoint does not
do symmetric RTP and symmetric RTCP, the media relay is unable to
perform its function if there is a symmetric NAT in the path.
"Symmetric NAT" is defined in section 5 of RFC3489 [2].
There are other instances where symmetric RTP and symmetric RTCP are
helpful, but not required. For example, if a firewall can expect
symmetric RTP and symmetric RTCP then the firewall's dynamic per-call
port filter list can be more restrictive compared to non-symmetric
RTP and non-symmetric RTCP.
4. Security Considerations
There is no additional security exposure if a host complies with this
specification.
5. IANA Considerations
This document doesn't require any IANA registrations.
6. References
6.1 Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
6.2 Informational References
[2] Rosenberg, J., Weinberger, J., Huitema, C. and R. Mahy, "STUN -
Simple Traversal of User Datagram Protocol (UDP) Through
Wing Expires April 7, 2005 [Page 4]
Internet-Draft Symmetric RTP and RTCP October 2004
Network Address Translators (NATs)", RFC 3489, March 2003.
[3] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[4] Camarillo, G., "The Alternative Network Address Types Semantics
for the Session Description Protocol Grouping Framework",
draft-ietf-mmusic-anat-01 (work in progress), June 2004.
[5] Schulzrinne, H., Casner, S., Frederick, R. and V. Jacobson,
"RTP: A Transport Protocol for Real-Time Applications", STD 64,
RFC 3550, July 2003.
[6] Srisuresh, P. and K. Egevang, "Traditional IP Network Address
Translator (Traditional NAT)", RFC 3022, January 2001.
[7] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol", RFC 2327, April 1998.
[8] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[9] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
[10] Andreasen, F. and B. Foster, "Media Gateway Control Protocol
(MGCP) Version 1.0", RFC 3435, January 2003.
[11] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[12] Sollins, K., "The TFTP Protocol (Revision 2)", STD 33, RFC
1350, July 1992.
[13] Rosenberg, J., "Traversal Using Relay NAT (TURN)",
draft-rosenberg-midcom-turn-05 (work in progress), July 2004.
Author's Address
Dan Wing
Cisco Systems
170 West Tasman Drive
San Jose, CA 95134
USA
EMail: dwing@cisco.com
Wing Expires April 7, 2005 [Page 5]
Internet-Draft Symmetric RTP and RTCP October 2004
Intellectual Property Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
Disclaimer of Validity
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
Wing Expires April 7, 2005 [Page 6]
Internet-Draft Symmetric RTP and RTCP October 2004
Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Wing Expires April 7, 2005 [Page 7]